Conditional expressions

CASE

Goes through conditions in sequence and returns the value of the result associated with the matching condition.

Syntax 1

CASE 
    WHEN condition THEN result 
    [ ... ]
    [ ELSE result ]
END

Parameters

Parameter	Description
condition	An expression that evaluates to a BOOLEAN value.
result	A value or an expression that evaluates to a value. The CASE expression returns result if its associated condition evaluates to true.

Returns

• The result associated with the first condition that evaluates to true. Or,
• The result in the ELSE clause if no condition evaluates to true. Or,
• NULL if there is no ELSE clause and no condition evaluates to true.

Syntax 2

CASE expression
    WHEN value THEN result 
    [ ... ]
    [ ELSE result ]
END

Parameters

Parameter	Description
expression	An expression that evaluates to a value. expression is computed in the first place and its value will be compared with value in the WHEN clause.
value	A value or an expression that evaluates to a value. Each value is a potential match for the expression.
result	A value or an expression that evaluates to a value. The CASE expression returns result if its associated value matches the expression.

Returns

• The result associated with the first value that matches the expression. Or,
• The result in the ELSE clause if no value matches the expression. Or,
• NULL if there is no ELSE clause and no value matches the expression.

Example

The following statement (using Syntax 1) classifies the distance of each trip in the table 'taxi_trips' into four levels.

SELECT id, distance,
  CASE 
    WHEN (distance < 3) THEN 'short'
    WHEN (distance >= 3 AND distance < 10) THEN 'mid'
    WHEN (distance >= 10 AND distance < 20) THEN 'long'
    WHEN (distance >= 20) THEN 'extra'
  END AS "Category"
  FROM taxi_trips;

 id | distance | Category 
----+----------+----------
  1 |       16 | long
  2 |       23 | extra
  3 |        6 | mid
  4 |        9 | mid

The following statement (using Syntax 2) classifies the distance of each trip according to its digits.

SELECT id, distance,
  CASE LENGTH (distance::VARCHAR)
    WHEN  1 THEN 'One-digit'
    WHEN  2 THEN 'Double-digit'
    WHEN  3 THEN 'Three-digit'
  END AS Digit
  FROM taxi_trips;

 id | distance |    Digit     
----+----------+--------------
  1 |       16 | Double-digit
  2 |       23 | Double-digit
  3 |        6 | One-digit
  4 |        9 | One-digit
(4 rows)

COALESCE

Returns the first non-null value or null if all values are null.

Syntax

COALESCE ( value [ , ... ] )

NULLIF

Returns null if value1 equals to value2, otherwise returns value1.

Syntax

NULLIF ( value1, value2 )